1. Field of Invention
The present invention relates to an automatic focusing method and a digital imaging device using the same, and more particularly to an automatic focusing method in a high noise environment and a digital imaging device using the same.
2. Related Art
With the development of digital cameras, photographing has been no longer a costly consumption. A user can take a desired image at will to record a moment or scenario worth memorizing. Generally, most cameras have the automatic focusing function in order to obtain an image with high definition when photographing.
With reference to FIG. 1a, in the prior art of focusing, corresponding digital images 10a,10b,10c,10d,10e,10f,10g,10h are captured at different object distances. The digital camera would calculate the high-frequency signal in the focusing frame for each digital image, and a curve graph for the objects distance and the high-frequency signals as shown in FIG. 1b is derived. Then the object distance corresponding to the maximum high-frequency signal, which is the optimum focusing object distance, is found out by using the conventional curve fitting and solving technology. Finally, the focusing group of the lens is moved to the optimum focusing position and focusing is thus accomplished.
However, when a photo needs to be taken under a high international standards organization (ISO) condition in an environment with low luminance, noises are likely to occur in the digital image. For the digital images captured under low contrast and high ISO conditions, the noise reduction effects of conventional de-noising algorithms are rather limited. Moreover, the details of the scenery in the digital image are often out of focus, so that the automatic focusing module in the digital camera fails to obtain an effective high-frequency signal in the case of extremely high noises. In this case, with the focusing technology in the prior art, a curve graph for the object distance and the high-frequency signal as shown in FIG. 1c tends to be resulted in, where no apparent global maximum exists, so that the focusing object distance determined by the automatic focusing procedure is not the most accurate one.